It is generally known to use light reflective targets in photogrammetric measurement of surfaces. According to this practice, the light reflective targets are positioned in a defined pattern on a surface to be measured and the positional relationship of the targets is measured and/or verified. Specifically, in a typical system, a measurement technician mounts multiple reflective targets on the surface to be measured and one or more cameras are then used to image the targets on a two-dimensional medium such as a film or a digital image sensor. A process of triangulation is then used to determine the relative three-dimensional positions of the targets. This permits development of a three dimensional image of the targeted surface. In the field of process control, such imaging may be used to compare the plotted positions of the targets on a formed part against a pre-established map of the specified formed part contours. This comparison is used to determine whether the formed part complies with given specification tolerances. One deficiency in such measurement systems is their dependence on manual placement of individual targets. In particular, in the event that the imaging technician fails to place targets properly the measurement will be in error.
Reflective targets for use in photogrammetric imaging are disclosed, for example, in U.S. Pat. No. 5,073,005 to Hubbs, having a filing date of May 2, 1988, and an issue date of Dec. 17, 1991. As best understood, this reference discloses target assemblies having a body with a mounted support surface adapted to hold a retro-reflective target. In one disclosed embodiment, the target body is elongated and provides a pair of target support surfaces at opposing ends. These target assemblies may be mounted on tooling or other surfaces for photogrammetric imaging evaluation. When using existing target assemblies, good results have been difficult to achieve due to difficulties in precisely aligning individual targets. Achieving good results may be particularly difficult when measuring surfaces such as boreholes and shaft surfaces.